The image above shows the characters themselves when they have actually activated their Bankai. By HNDRNT26.

Ikkaku Madarame
Madarame  is the 3rd Seat of the 11th Division, with Captain Zaraki Kenpachi. Ikkaku’s Bankai has only been revealed a few times. Madarame’s Bankai, Ryumon Huzukimaru has three over-sized weapons connected by an equally large chain. To activate it during his battle with Edrad, Ikkaku slams the two broken pieces of his Shikai back into the spear-form and says Bankai. Despite all three weapons being of near-ridiculous proportions, Ikkaku wields them easily. Ikkaku usually holds one blade in each hand and lets the central part “float” behind him, though he will occasionally spin the entire configuration about using the center handle as a pivot.

Toshiro Hitsugaya
Hitsugaya is the Captain of the 10th Division with Rangiku Matsumoto being her Lieutenant. Toshiro’s Bankai has been revealed quite a few times during fights and movies. Therefore we’ve gotten used to his ice attacks and angel ice wings. His Bankai, Daiguren Hyorinmaru has the ability to cause ice to flow from Hyōrinmaru to Hitsugaya, starting at his right arm which ice forms with the shape of a dragon’s head around his sword hand, encasing the sword up to the hilt; which also changes from the shape of a four-pointed star to that of eight-pointed star. The ice continues forming up over his shoulders with two large wings sprouting from his back and a long tail. The ice forms down his left arm encasing his hand which ends in a claw. Hitsugaya’s feet are encased in ice in a similar way to his left hand which as they to end in claws.

Ichigo Kurosaki
Ichigo’s Bankai has changed dramatically within the few new manga Chapters. As Ichigo is the main character, he’s Bankai has been seen by most people.  During the “Last Substitute Shinigami” Arc, Ichigo’s Bankai changed, and got an even more hectic looking Bankai. He wears a white undershirt beneath his black shihakusho, which is now open. Held together by three “x”-shaped marks, it ends in a number of coattails. Ichigo wears black gloves with white “x”-shaped marks appear on both the gloves and his forearms. Tensa Zangetsu’s blade is now more curved as it nears the tip, with three protrusions along the blunt end of the sword close to its tip. While retaining its manji-like shape, the tsuba becomes more curved and jagged. Tensa Zangetsu’s chain is now longer, attaching to both the base of the sword’s hilt and to Ichigo’s glove, forming a wide loop. It’s too early to say if he’s got any extra powers.

Byakuya Kuchiki
Byakuya is the captain of the 6th Division with Renji Abarai as his Lieutenant. Byakuya’s Bankai Senbonzakura Kageyoshi is a larger version of his Senbonzakura Shikai. Byakuya drops his sword straight down. The sword phases into the ground, as though it were a pool of water. Energy ripples expand out, turning the surrounding area dark, and within seconds, two rows of a thousand giant blades rise up from the ground. Those thousand swords would scatter and countless flying blades pierce the enemy. No one knows the exact number of blades generated and no one can see the movements of those blades or even dodge them. The number of blades is great enough that Byakuya can use them for defense as well as offense at the same time. He commonly forms them into large masses, designed to obliterate opponents instantaneously. As it moves through the air like torrents of waves, it gives the appearance of cherry blossoms flowing swiftly along with the wind.

Renji Abarai
Renji is the Lieutenant of the 6th Division. His Bankai has been seen by most other Soul Reapers as he rushes into defending other people with his powers. Renji’s Bankai Hihio Zabimaru transforms into a into a massive version of its Shikai form resembling a skeleton of a snake. Zabimaru gains many more segments, which are much larger resembling a mix between a snake’s vertebral column and the pick-like protrusions of his Shikai segments, and it also gains a snake-like head about the size of a small car. Renji is also slightly transformed, gaining a fur cowl around his neck. The cowl appears to be the fur of an ape and thus, the ape’s skull could be seen over Renji’s left shoulder while the rest of the fur extends to cover his right arm. Zabimaru itself, in Bankai form, does not literally “cut;” Renji usually catches his opponent with the snake-head of his weapon and smashes his opponent into the ground with it. The head has also proven capable of inflicting heavy damage through biting.

If you read all that, I’m sure you’ve learned something new! Source: Bleach.Wikia.com.